A TSV is a vertical electrical connection passing completely through a silicon wafer or die. TSV technology is important in creating 3D packages and 3D integrated circuits (IC). It provides interconnection of vertically aligned electronic devices through internal wiring that significantly reduces complexity and overall dimensions of a multi-chip electronic circuit.
A typical TSV process involves forming TSV holes and depositing conformal diffusion barrier and conductive seed layers, followed by filling of TSV holes with a metal. Copper is typically used as the conductive metal in TSV fill as it supports high current densities experienced at complex integration, such as 3D packages and 3D integrated circuits, and increased device speed. Furthermore, copper has good thermal conductivity and is available in a highly pure state.
TSV holes typically have high aspect ratios making void-free deposition of copper into such structures a challenging task. CVD deposition of copper requires complex and expensive precursors, while PVD deposition often results in voids and limited step coverage. Electroplating is a more common method of depositing copper into TSV structures; however, electroplating also presents a set of challenges because of the TSV's large size and high aspect ratio.
In a typical TSV electrofilling process, the substrate is contacted with a plating solution which includes copper sulfate as a source of copper ions, sulfuric acid for controlling conductivity, chloride ion to enhance suppressor adsorption and several other additives. However, the use of standard commercially available electrolytes often results in very slow plating and in formation of voids during TSV filling. For example, a typical electrolyte is prepared by combining a solution of copper sulfate, which is supplied at Cu2+ concentration of less than 65 g/L with concentrated or 50% concentrated sulfuric acid. Pre-mixed electrolytes containing both copper salt and an acid are also available, however they typically have Cu2− concentrations of less than 60 g/L. In both cases, the commercially available solutions are prepared at such concentrations so as to avoid precipitation of copper salts at shipping and storage temperature of between about 0-10° C.